The molten salt electrolyzer often contains in the inside thereof a highly reactive or toxic molten salt as an electrolytic bath, and the electrolyzer is made ready for electrolysis by forming a closed space and heating the electrolytic bath to melt the salt material. The judgment as to whether the electrolyzer is ready for electrolysis as a result of completion of the melting of the electrolytic bath is made by an operator based on the electrolyzer temperature information and other information and based on his/her own experience. The electrolytic bath has a high melting point and occurs as a solid at ordinary temperature. Generally, the gaseous phase section in the electrolyzer is divided into an anode compartment or chamber and a cathode compartment or chamber by insertion of a partition wall into the electrolytic bath. The electrolytic bath may solidify in a state of unbalance between the anode chamber and cathode chamber according to the pressure conditions in the electrolyzer in the process of solidification of the electrolytic bath. In some instances, even when the electrolytic bath in such state is remelted, the liquid level unbalance remains undissolved and it is difficult to carry out electrolysis safely.
An example of this type of molten salt electrolyzer is described in Japanese Patent Laid-Open Application (JP Kokai) No. 2002-339090 (Patent Document 1). The electrolyzer described in Patent Document 1 is a fluorine gas generator for generating highly pure fluorine gas by electrolysis of a hydrogen fluoride-containing mixed molten salt and comprises an electrolytic cell divided into an anode chamber and a cathode chamber by means of a partition wall, and pressure maintenance means for maintaining the pressures in the anode chamber and cathode chamber at a predetermined level through gas feeding to and/or gas discharging from the anode chamber and cathode chamber. The bath liquid surface in the electrolyzer is maintained in an equilibrium state by the pressure maintenance means during steady electrolytic operation.
Meanwhile, on the occasion of stopping the operation of the fluorine gas generator, the inlet and outlet of the electrolyzer are first closed, and the electrolytic operation is then stopped. Generally, a carbon electrode is employed as the anode of the electrolyzer. The fluorine gas remaining in the anode chamber is adsorbed on this carbon electrode and the pressure in the anode chamber decreases accordingly, with the result that the bath liquid surface in the anode chamber arises as compared with the level in the cathode chamber, bringing about an unbalanced state. In stopping the fluorine gas generator, the heating of the electrolyzer is also stopped and, therefore, as the temperature lowers, the electrolytic bath solidifies while maintaining that liquid level unbalance.
As mentioned above, the electrolyzer is operated for electrolysis while melting the electrolytic bath by heating in a closed space, and the judgment as to whether the electrolyzer is ready for electrolysis as a result of completion of the melting of the electrolytic bath is made by an operator based on the electrolyzer temperature information and other information and based on his/her own experience. The electrolyzer temperature information consists of the results of temperature measurements at parts of the electrolytic bath contained in the electrolytic cell and weighing several hundred kilograms to several tons. Therefore, it is possible that the electrolytic bath is not yet in a completely molten state due to insufficient heating and/or thermal insulation and, in such case, in particular when the bath remains solid around one or both electrodes, the passage of electric current is impossible. Even when the bath is in a partially molten state around the electrodes, the materials for electrolysis in the electrolytic bath begin to be consumed with the start of electrolysis and the electrolytic bath around the current-carrying portions begins to change in composition to the higher melting point side. At worst, the melting point arrives at a level exceeding the limitations of the heating means of the apparatus and the bath precipitates out on the electrode surface. Once placed in such a state, it is also very difficult to restore the normal state by melting the solidified electrolytic bath again. Therefore, it is very important to confirm the state of melting of the electrolytic bath prior to starting electrolysis. For realizing this, it is necessary to open the lid or covering of the electrolyzer. However, the molten salt contained in the electrolyzer is highly reactive and toxic, hence it is undesirable to open the electrolyzer while the electrolytic bath is in a molten state. In addition, there is a fear that some or other impurity or impurities may enter the electrolyzer on the occasion of opening, serving as a factor in decreasing the purity of the product or products. It is in reality difficult to open the electrolyzer for confirming the state of the inside. Thus, the advent of a control method by which judgment can be made as to whether the bath is in a sufficiently molten state without opening the electrolyzer is awaited for safely operating such molten salt electrolyzer.
If when the bath surface is in a solidified state in an unbalanced condition, electrolysis is started while the unbalance is not yet dissolved on the occasion of remelting, the electrodes are partly placed under abnormal load conditions because of the electrolysis conditions differing from the normal ones. Further, when the electrolytic bath liquid level unbalance is found in the vicinity of the lower end of the partition wall separating the anode chamber and cathode chamber from each other, the possibility of the gases generated in the anode chamber and cathode chamber, respectively, mixing with each other becomes high and, in particular in electrolytic fluorine generation, explosion will happen if fluorine generated from the anode and hydrogen generated from the cathode mix with each other in the gaseous phase. This explosion may damage the carbon anode supported in the electrolyzer or the electrolyzer itself. For these reasons, a control method is demanded by which the electrolytic bath levels can be balanced so that electrolysis can be safely restarted after remelting the electrolytic bath in the molten salt electrolyzer.
The present invention, which has been made in view of the problems discussed above, has for its object to provide a control apparatus or system and a control method by which the transition from the bath melting step to the state allowing the start of electrolysis in a molten salt electrolyzer can be safely achieved.